Radio waves—i.e., electromagnetic radiation—are widely used to communicate information from place to place. Information is modulated onto a radio-frequency carrier, which is emitted or radiated from one antenna and travels through intervening materials to reach a second antenna, where the signal is amplified and demodulated to recover the original information. (Some intervening materials may impede or prevent the propagation of radio waves, while others have little effect.)
Radio transmission power generally follows an inverse-square law, so that transmission over twice a given distance requires four times the power (or a receiver that is four times as sensitive). However, a directional antenna can improve this situation somewhat, by concentrating radiated power in one direction at the expense of other directions. (The same antennas are often used for transmission and reception, and are constructed so that improved directional radiation during transmission is accompanied by improved reception of signals arriving from the same direction.) Since increasing radio power is correlated with increasing design, manufacture and operational cost, directional antennas offer an economically favorable way to increase communication range for point-to-point and point-to-multipoint connections (as compared to ordinary broadcast operations). FIG. 2 represents the horizontal radiation pattern of a typical directional antenna (“Yagi” antenna 210). Heavy, multi-lobed pattern 220 indicates the power radiated on transmission (or the sensitivity during reception) at various points in a circle around the antenna. The pattern has a maximum of 100% or 0 dB in the principal direction (0°, reference character 230), and quickly falls off in either direction. The beam angle 240 may be set (by definition) where the power/sensitivity is 3 dB below the maximum (dashed circle 250); around most of the circle, the power/sensitivity may be reduced by 6 dB or more (260). The beam angle in this example is about 45° (22.5° in each direction). Opposite the principal direction (180°, reference character 270), very little power may be emitted, and signals arriving from that direction may not be detected. A similar shape can be plotted to show the radiation pattern when viewing the antenna from the side. The lobes (and particularly the principal lobe) are thus shaped somewhat like teardrops or spherical cones.
At radio frequencies where phase control of the modulated signal is possible, multiple spaced-apart antennas can be operated together to achieve directional control of an anisotropic radiated beam. Phase differences between the signals at each antenna create an interference pattern that has a controllable shape and direction—a shape similar to the lobe pattern described above, whose principal axis can also be reoriented. The beam direction can be changed as quickly as a single carrier period—mere nanoseconds for a 2.5 GHz signal—and much faster than any physical antenna can be pointed. A line of phase-controlled antennas can sweep a wedge or halo of radio energy from perpendicular to the line, to one side or the other of the perpendicular direction. A two-dimensional array of phase-controlled antennas, such as the one depicted in FIG. 3 at 310 can form a conical beam of energy 320 and point it at directions skewed 330 from the normal to the array 340.
The width of the wedge, halo or beam is generally inversely proportional to the number of antennas in the line or array: more phase-controlled emitters can produce a narrower angle beam. The “width” of the beam is generally considered to be the angle to the side of the beam where the radiated power (or reception sensitivity to signals from that direction) has declined to a particular fraction of the maximum power at the “center” of the beam. Decibels are often used to express this ratio. Thus, for example, a beam might be defined as the conic or teardrop section over which the power is within 3 dB of the maximum power.
A phased-array antenna cannot direct its beam in an arbitrary direction—it becomes less efficient as the beam angle diverges from the principal direction (i.e., perpendicular to the line or plane of the array). For antennas with a reasonable number of emitters, the useful steering range may be about ±30° from the main direction. Thus, to achieve directional control over a full 360° (in one plane), a transceiver might need six antenna arrays (and many more for beam control over a hemispherical or full spherical range).
Antenna arrangements that can provide anisotropic control of transmission energy and reception sensitivity over a wider range than a prior-art phased array may be of significant value in this field.